The present invention is directed to label and tape constructions that employ multilayer adhesives, in which at least the layer furthest from the facestock is a rubber-based, tackified pressure-sensitive adhesive (PSA). The constructions are characterized by reduced diffusion or migration of liquid tackifiers, plasticizers and other low molecular weight constituents between layers.
Adhesive tapes and labels having a laminate construction are well known. In a typical construction, one or more layers of adhesive are coated on or otherwise applied to a release liner, and then laminated to a facestock, such as paper, polymeric film, or other flexible material. The adhesive may be a pressure-sensitive adhesive (PSA), and may be rubber-based or acrylic-based. Rubber-based hot melt PSAs (HMPSAs) typically contain one or more natural or synthetic elastomers, tackified with a petroleum resin, rosin or rosin derivative, and/or other ingredients, such as plasticizers, which improve the tack of the adhesive.
During label manufacture, a laminate of a facestock, PSA layer and a release liner is passed through apparatus that converts the laminate into commercially useful labels and label stock. The processes involved in the converting operation include printing, slitting, die-cutting and matrix-stripping to leave labels on a release liner, butt-cutting of labels to the release liner, marginal hole punching, perforating, fan folding, guillotining and the like.
Die-cutting involves cutting of the laminate to the surface of the release liner. Hole punching, perforating and guillotining involve cutting clean through the label laminate.
The cost of converting a laminate into a finished product is a function of the speed and efficiency at which the various processing operations occur. While the nature of all layers of the laminate can impact the cost of convertibility, the adhesive layer typically has been the greatest limiting factor in ease of convertibility. This is due to the viscoelastic nature of the adhesive, which hampers precise and clean penetration of a die in die-cutting operations and promotes adherence to die-cutting blades and the like in cutting operations. Stringiness of the adhesive also impacts matrix-stripping operations, which follow die-cutting operations.
Achieving good convertibility does not, by necessity, coincide with achieving excellent adhesive performance. Adhesives must be formulated to fit specific performance requirements, including sufficient shear, peel adhesion, and tack or quick stick, at various temperatures. A good, general purpose adhesive may exhibit poor convertibility simply because the adhesive is difficult to cleanly cut. Such an adhesive may stick to a die or cutting blade. An ideal HMPSA would have both high tack and high cohesive strength, good flow characteristics if it is to be used in the bulk state, so that it can be coated or otherwise applied to a facestock, or coated on a release liner and laminated to a facestock, and good converting performance.
In an effort to address the sometimes conflicting demands of good adhesive performance and good convertibility, multilayer constructions having two or more layers of adhesives have been proposed and described.
U.S. Pat. No. 4,260,659 to Gobran, incorporated herein by reference, describes a multilayer PSA tape formed of a plurality of superimposed PSA layers, the outer layer of which is softer than the immediate underlying layer.
U.S. Pat. No. 4,894,259 to Kuller, incorporated herein by reference, describes a process for producing a PSA tape comprised of a plurality of concurrently coated layers, at least the outer layer of which is a PSA layer, with contiguous layers defining an interphase between each layer the interphase comprising a photopolymerized matrix of polymer chains extending from the matrix of one layer to the interface to the matrix of a contiguous layer. When photopolymerized the layers cannot be delaminated.
WO96/08367 to Ercillo et al., incorporated herein by reference, describes a multilayer PSA construction in which one layer is formed with a first adhesive having a first glass transition temperature and a second layer having a second glass transition temperature. The first glass transition temperature differs from the second glass transition temperature. To the extent a tackifier and/or plasticizer is incorporated in such multilayer constructions there is no specific instruction on how to control diffusion or migration of tackifiers and/or plasticizers.
WO96/08369 to Ugolick et al, incorporated herein by reference, describes a multilayer PSA construction in which a first layer is a barrier layer and a second layer is an adhesive layer on the barrier layer. The barrier layer is a pressure-sensitive adhesive and inhibits the migration of mobile species such as oils, resins, and tackifiers from the adhesive layer into the facestock. In one embodiment, the adhesive layer is an apolar rubber-based composition, and the barrier layer is a thermoplastic acrylic PSA. The construction is usefull as a label, and is designed to improve adhesion and preserve the appearance of the label by inhibiting wrinkling and/or staining.
U.S. Pat. No. 5,290,842 to Sasaki et al, incorporated herein by reference, discloses a PSA construction that utilizes a mutually immiscible first elastomerxe2x80x94typically a styrenexe2x80x94butadiene (SB) and/or styrene butadiene-styrene (SBS) block copolymerxe2x80x94exhibiting a first glass transition temperature, and second elastomerxe2x80x94typically a styrene-isoprene (SI) and/or styrene-isoprene-styrene (SIS) block copolymerxe2x80x94exhibiting a second glass transition temperature greater than the first. The composition is tackified by a tackifying system comprising a tackifier that is preferentially miscible in the second elastomer to provide a composition exhibiting excellent processibility and ambient and low temperature properties.
One problem with multilayer adhesive constructions is diffusion and migration of tackifiers and plasticizers, especially those of low molecular weight, which causes an undesirable change of adhesive properties. Although an intermediate barrier layer can be positioned between adhesive layers, such an approach complicates the manufacturing process and increases the cost of the multilayer construction. One proposal for reducing tackifier migration between layers has been to provide substantially equal levels of liquid tackifiers and/or plasticizer oil additives to the two layers to prevent diffusion or migration of such materials from one layer to another. However, maintaining the same ratio of tackifiers and plasticizers to elastomers in two adhesive layers has not led to a completely satisfactory result.
A need clearly exists for a more effective means of preventing migration of liquid plasticizers and tackifiers between layers of multilayer adhesive constructions so as to retain the properties of the individual adhesive layers for extended periods of time. There is also a need to prevent or reduce tackifier migration into polyolefin facestocks. When a hot melt PSA formulated with rubber-based block copolymers is used with a polyolefin facestock, swelling and hardening of the facestock can occur over time. This manifests itself as a wrinkling of the adhesive construction and/or a loss of adhesion, and is believed to be caused by the migration of tackifiers and plasticizers utilized in the rubber-based adhesives into the facestock.
According to one aspect of the present invention, there are provided multilayer adhesive constructions that exhibit reduced migration of tackifiers between adhesive layers and reduced migration into the facestock, while providing good adhesion to a variety of substrates. The constructions exhibit good adhesion, excellent convertibility in label and linerless label manufacture, and good low temperature performance. In another aspect of the invention, adhesive constructions having polyolefin facestocks and one or more layers of tackified PSAs are provided, and exhibit reduced tackifier migration into the facestocks in consequence of the particular tackifier(s) employed.
In one embodiment of the invention, a multilayer adhesive construction comprises a laminate of (a) a face stock, (b) a first adhesive layer, preferably coated on or contiguous with the facestock and (c) a second adhesive layer, preferably coated on or contiguous with the first adhesive layer. A release liner may protect the adhesive until it is to be applied to a substrate. Alternatively, a linerless construction is made, with the backside of the facestock having a release material (e.g., silicone) coated thereon.
The first adhesive layer is a tackified blend of at least two immiscible elastomers. A first elastomer is a first diene-containing elastomer characterized by a first glass transition temperature, Tg1, and a first value of tangent delta, tan xcex41. A second elastomer is a second diene-containing elastomer characterized by a second glass transition temperature, Tg2, and a second value of tangent delta, tan xcex42, where Tg1 less than Tg2. The two immiscible elastomers are provided in relative proportions such that the first elastomer (having the lowest Tg) is a continuous phase, and the second elastomer (having a higher Tg) is a discernible discontinuous phase distributed throughout and contained within the continuous phase of the first elastomer.
The first adhesive layer is tackified with one or more tackifiers and, optionally, contains one or more plasticizers. The tackifiers (and optionally the plasticizers) are selected to be preferentially soluble in the second elastomer (which has the higher Tg).
The second adhesive layer is a tackified elastomer (or tackified mixture of elastomers) that is a diene-containing elastomeric composition different from the first adhesive layer. In one embodiment, this diene-containing elastomeric composition is formed of a single elastomer or a miscible blend of elastomers that form a single visually discernible phase. One or more tackifiers are dispersed throughout the phase. In another embodiment, the elastomeric composition is formed of a continuous phase of a first elastomer and discontinuous phase of a second elastomer, and one or more tackifiers (and, optionally, one or more plasticizers) are dispersed throughout, and preferentially soluble in, the continuous phase.
Additional low molecular weight components, such as extenders, fillers, pigments and stabilizers may be present in the first and/or second adhesive layer(s). The adhesive layers can be independently extended using extender oil, such as Shellflex 371, and/or fillers, such as calcium carbonate. The extenders can, and preferably are, present in both the continuous and discontinuous phases of each adhesive layer. The adhesive layers also can be extended using the low molecular weight segment of the tackifiers and/or plasticizers.
In a preferred multilayer adhesive construction, the continuous phase of the first adhesive layer is formed of one or more butadiene-containing elastomers, such as styrene-butadiene-styrene (SBS) and/or styrene-butadiene (SB) block copolymers, with a mixture of SBS and SB block copolymers being preferred. Contained within the continuous phase is a discernible, discontinuous phase formed of one or more isoprene-containing elastomers, such as styrene-isoprene (SI) and/or styrene-isoprene-styrene (SIS) block copolymers, and/or multiarmed styrene-isoprene (SI)x block copolymers. One or more tackifiersxe2x80x94each preferentially soluble in the discontinuous phasexe2x80x94as well as one or more plasticizers and other low molecular weight components, are added to the elastomers forming the first adhesive layer. The second adhesive layer of the preferred construction comprises a continuous phase of an isoprene-based elastomer, such as an SIS block copolymer, and/or SI block copolymers, or a mixture of SB and SIS and/or SI elastomers, with a weight ratio of butadiene-based elastomers to isoprene-based elastomers less than about 0.5. One or more tackifiers are present in the second adhesive layer. Optionally, one or more plasticizers and other low molecular weight components are present.
Migration of tackifiers between the first and second adhesive layers is inhibited by the tackifiers"" preferential solubility in the discontinuous phase of the first layer and the continuous phase of the second layer. Consequently, the adhesive properties and convertibility characteristics of the adhesive construction are preserved for extended periods of time. The result is a more commercially appealing product.
In addition to being a barrier to migration of tackifiers and other components, the first adhesive layer can be used to control the converting characteristics of the overall construction, while the second adhesive layer can be keyed to provide desirable overall adhesive properties. Also, in one embodiment of the invention, the facestock can be reinforced by selecting at least one adhesive layer that provides structural reinforcement. The phase separated layer(s) can be used to this end. Consequently, lower caliper, less expensive facestock can be used without sacrificing structural integrity of the overall laminate construction. In another embodiment, improved label repulpability can be targeted by including at least one repulpable PSA layer in the construction.